Integrated circuits ("ICs") have many uses in industries ranging from communications to consumer electronics. To this end, various types of ICs are commonly designed for use in conjunction with other circuit components mounted on a common printed circuit ("PC") board.
By way of example, a power transistor IC is formed by fabricating one or more transistor cells on a silicon wafer, commonly referred to as a transistor "chip". The transistor chip is attached to an insulating layer, normally a ceramic substrate, which is thermally, but not electrically, conductive. The ceramic substrate is itself attached to a thermally conductive mounting flange. A protective cover is secured to the flange, covering the substrate and transistor chip, thereby forming a power transistor "package."
Various electrically conductive (e.g., thin metal) leads may be attached to, and extend away from, the power transistor package (i.e., extending outside the protective cover) to connect common terminals of the transistor chip to external circuit elements. For example, for a bipolar junction type power transistor, respective electrical leads attached to the package are connected to a base, emitter and collector of the transistor chip.
Finished IC component packages are typically mounted on a PC board as part of an assembly process. Because most IC packages, and in particular power transistor packages, generate a significant amount of heat during operation, the bottom surface of the package mounting flange is typically affixed to a heat sink located beneath the surface of the PC board. For example, a single layer PC board--i.e., where there is a single layer of dielectric material between respective top and bottom surfaces of the PC board,--typically has a metallic bottom surface that is connected with screws or solder to a metallic heat sink surface used to mount one or more IC packages, wherein various surface leads on the PC board are thereafter connected to respective leads extending from the package, e.g., by a solder lead or a clamp.
There are several known techniques for securing an IC package to a heat sink surface. For example, as illustrated in FIG. 1A, an exemplary IC package 20 may be secured to a heat sink 22 by a screw 24 located on each end of a mounting flange 26. However mounting arrangement requires a manual step in the PC board assembly process, which adds considerable cost. Further, it is difficult to apply a uniform force across the entire bottom surface of the mounting flange 26 with the screws 24, which tend to exert greater force at the screw locations than over the rest of the flange bottom.
Referring to FIG. 1B, another common technique is to solder (or glue) the mounting flange 26 to the heat sink surface 22. While this process is more easily automated, the solder or bonding material 28 will invariably have a different thermal expansion coefficient than the respective (typically metal) mounting flange 26 and heat sink 22. As a result, the bond between the mounting flange 26 and heat sink 22 will weaken or even be destroyed by the thermal expansion stress between the respective layers, especially when subjected to repeated changes in temperature during each use of the IC package 20. Further, the presence of the intervening bonding material layer 28 may lesson the effectiveness of the heat conduction between the flange 26 and heat sink 22. A still further disadvantage with this approach is that, in order to remove an IC package for repair or replacement, the entire heat sink surface must be heated to break the solder bond, thereby causing all other solder bonds on the same heat sink to be weakened.
In particular, it is very desirable that a uniform force be applied across the bottom surface of the mounting flange 26 for the most efficient conduction of heat to the heat sink 22. Further, because the greatest amount of heat will typically flow through the center area of the package and mounting flange, i.e., below the active transistor cell area, it is desirable to apply the greatest force against the heat sink 22 in the center of the package 20.
To this end, as seen in FIGS. 1C and 1D, still another known IC package mounting arrangement involves the use of one or more screws 30 inserted into, and protruding above, the surface of the heat sink 22. A resilient metal strip 32 is extended from the screw(s) 30 and is shaped so as to apply a clamping force upon the cover of the IC package 20, thereby distributing a substantially centered force that "secures" the mounting flange 26 against the heat sink 22. However, this approach still requires the use of a labor intensive step in the PC board assembly process.
Thus, it would be desirable to provide an improved mounting arrangement for securing an IC component package to a heat sink, such as a heat sink attached to a bottom of a PC board, whereby a substantially uniform pressure is exerted for highest heat transfer efficiency, while still allowing for automated PC board assembly.